Illegal reflective access by org.jenkinsci.utils.process.ProcessUtils to field java.lang.ProcessImpl#pid

The ProcessUtils in lib-process-utils allows getting PID from a process using reflection. As part of the general Java 9/10/11 hackathon all setAccessible reflection use was being reviewed.

This issue is to remove the reflection use, and as a nice side effect the code will work on Windows too. However, this requires upgrading to Java 9+, so I don't really expect it to happen anytime soon. For now I wanted to preserve the little bit of work I did.